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giant star
A giant star is a star with substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature.〔Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moore, New York: Oxford University Press, 2002. ISBN 0-19-521833-7.〕 They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III.〔giant, entry in ''The Facts on File Dictionary of Astronomy'', ed. John Daintith and William Gould, New York: Facts On File, Inc., 5th ed., 2006. ISBN 0-8160-5998-5.〕 The terms ''giant'' and ''dwarf'' were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.
Giant stars have radii up to a few hundred times the Sun and luminosities between 10 and a few thousand times that of the Sun. Stars still more luminous than giants are referred to as supergiants and hypergiants.
A hot, luminous main-sequence star may also be referred to as a giant, but any main-sequence star is properly called a dwarf no matter how large and luminous it is.〔Giant star, entry in ''Cambridge Dictionary of Astronomy'', Jacqueline Mitton, Cambridge: Cambridge University Press, 2001. ISBN 0-521-80045-5.〕
== Formation ==
A star becomes a giant star after all the hydrogen available for fusion at its core has been depleted and, as a result, leaves the main sequence.〔 The behaviour of a post-main-sequence star depends largely on its mass.
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